Hydroxyapatite has excellent affinity to living body and excellent adsorptivity and hence have been studied as to utilities thereof in a variety of fields, particularly use thereof as implants for replacing or repairing hard tissues of living bodies. The implants require to have a biodynamic strength in addition to the affinity to living body. Nevertheless, hydroxyapatite is not satisfactory in terms of the strength even in the form of a sintered product. Accordingly, from the practical viewpoint, it is favorable to use a substrate or core material such as metallic materials, ceramics, glass, etc. and to form a hydroxyapatite coating film on the surface of said substrate or core material.
Various methods have hitherto been proposed for forming a hydroxyapatite coating film, for example, a thermal plasma spray method (cf. Japanese Patent First Publication (Kokai) No. 82893/1977), a spattering method (cf. Japanese Patent First Publication (Kokai) No. 109049/1983), a physical vapor deposition (PVD) or chemical vapor deposition (CVD) method (cf. Japanese Patent First Publication (Kokai) No. 111753/1984), an electrophoretic method (cf. Japanese Patent First Publication (Kokai) No. 128190/1978), a coating method (cf. Japanese Patent First Publication (Kokai) No. 118411/1978).
However, the thermal plasma spray method, spattering method, CVD method and PVD method are hardly applied to a substrate having a complicated shape, for example, onto the inner surface of porous substrate, and the electrophoretic method can not form the coating film onto a substrate having no electrical conductivity. On the other hand, the coating method is advantageously easy in the treatment, and the above-mentioned Japanese Patent First Publication (Kokai) No. 118411/1978 discloses a method for forming a coating film by suspending fine particles of apatite in water and coating the aqueous suspension onto the surface of a substrate, followed by calcining the coated layer. However, this coating method has still a problem that it is usually difficult to prepare very fine particles and the particles are easily agglomerated, and further, the particles dispersed in water have usually a particle size of more than 0.5 .mu.m, and hence, the apatite has less adhesion force to the substrate surface and the coating film is easily peeled off from the substrate.